The present invention relates to a zoom lens suitably used for a video camera using an imaging device, etc., and having a wide angle of view of 60xc2x0 or more, a high zoom ratio of about 10 times and a reduced full length.
Examples of conventional zoom lenses include a zoom lens described in Publication of Japanese Patent Application No. (Tokugan Hei) No. Hei-8-114092. Hereinafter, one example of the above-mentioned conventional zoom lens will be explained with reference to the drawings. FIG. 35 is a view showing a configuration of a zoom lens for a video camera of the prior art.
A zoom lens shown in FIG. 35 includes a first lens group 351 as a light beam condenser, a second lens group 352 as a variable power, a third lens group 353 as a light beam condenser, a fourth lens group 354 as a focus portion, an equivalent glass plate 355 corresponding to a quartz filter, a face-plate of an image pick-up device, or the like, and an image plane 356.
The first lens group 351 fixed with respect to the image plane 356 is in charge of an image pick-up action. The second lens group 352, moving forward and backward on an optical axis, changes the focal distance of the entire lens system by changing the magnifications. The fixed third lens group 353 is in charge of an action of condensing divergent light beams generated by the second lens group. The fourth lens group 354, moving forward and backward on the optical axis, is in charge of a focusing action.
Furthermore, the change of the image plane position due to the movement of the second lens group 352 during zooming is corrected so that an image is formed at a constant position by the movement of the fourth lens group 354, thereby keeping the image plane at a constant position.
Recently, in the video camera market, downsizing of an image pick up device and lens system has been strongly demanded. Furthermore, recently, it is an important requirement that electronic still cameras, which emerge in the market with the popularity of multimedia personal computers, are cheap and small.
However, in the above-mentioned conventional zoom lens, the number of lenses is ten and the zoom ratio of about 10 times is high, however, the full length of the zoom lens is relatively long with respect to the effective aperture of the lens positioned closest to the side of an object. Such a zoom lens does not have a compact size and cannot satisfy the demands of downsizing and high performance. Moreover, in the conventional methods for designing the zoom lens, it was difficult to satisfy all demands, namely, a large aperture, high magnification, a small size, and high degree of resolution.
It is therefore an object of the present invention to solve the above-mentioned problems of the conventional zoom lens and to provide a zoom lens having a compact size and wide angle of view, namely, a zoom lens having a few lenses, a zoom ratio of about 10 times, an angle of view of approximately 60xc2x0 or more, and to provide a video camera and an electronic still camera using the zoom lens.
According to the present invention, a zoom lens includes a first lens group having a positive refractive power and being fixed with respect to an image plane, a second lens group having a negative refractive power and being in charge of a variable power action by being moved on an optical axis, a third lens group having a positive refractive power and being fixed with respect to the image plane, and a fourth lens group having a positive refractive power and moving on the optical axis so that it keeps the image plane following up the movement of the second lens group and the object at a constant position with respect to a standard plane, wherein the first lens group includes, in the order of an increasing distance from the object, a lens having a negative refractive power, a lens having a positive refractive power, and a meniscus lens having a positive refractive power and having a convex surface facing the object side; the second lens group includes, in the order of an increasing distance from the object, a lens having a negative refractive power, a biconcave lens having a negative refractive power, and a lens having a positive refractive power, having a convex surface facing the object side, and being connected to the biconcave lens; the third lens group includes, in the order of an increasing distance from the object, a biconvex lens having a positive refractive power, a lens having a positive refractive power and having a convex surface facing the object side, and a lens having-a negative refractive power, wherein the lens having a negative refractive power of the third lens group is connected to the lens having a positive refractive power and having a convex surface facing the object side of the third lens group, the lens having a negative refractive lens of the third lens group has a concave surface facing the side of the image plane, and the connected lenses as a whole have negative refractive power; the forth lens group includes one lens; each of the second lens group, the third lens group and the fourth lens group contains at least one aspherical surface; and the following relationships are satisfied: 2.0 less than L/CL1 less than 2.3, 0.2 less than f4/L less than 0.35, and 0.3 less than fw/f4 less than 0.4, where CL1 is an effective aperture of the lens positioned closest to the object in the first lens group, L is a distance between the lens positioned closest to the object in the first lens group and the image plane, f4 is a focal distance of the fourth lens group, and fw is a focal distance at a wide-angle end.
According to the above-mentioned zoom lens, a high magnification, compact and wide-angle zoom lens having its aberration adequately corrected can be obtained with a simple configuration. In addition, the sufficient back focus and wide angel of view ca be obtained. Moreover, the full length of the lens can be reduced.
Next, a second zoom lens of the present invention includes, in the order of an increasing distance from an object, a first lens group having a positive refractive power and being fixed with respect to an image plane, a second lens group having a negative refractive power and being in charge of a variable power action by being moved on an optical axis, a third lens group having a positive refractive power and being fixed with respect to the image plane, and a fourth lens group having a positive refractive power and moving on the optical axis so that it keeps the image plane following up the movement of the second lens group and the object at a constant position with respect to a standard plane, wherein the first lens group comprises, in the order of an increasing distance from the object, a lens having a negative refractive power, a lens having a positive refractive power, and a meniscus lens having a positive refractive power and having a convex surface facing the object side; the second lens group comprises, in the order of an increasing distance from the object, a lens having a negative refractive power, a biconcave lens having a negative refractive power, and a lens having a positive refractive power, having a convex surface facing the object side, and being connected to the biconcave lens; the third lens group comprises, in the order of an increasing distance from the object, a biconvex lens having a positive refractive power, a lens having a positive refractive power and having a convex surface facing the object side, and a lens having a negative refractive power, wherein the lens having a negative refractive power of the third lens group is positioned with an air space with respect to the lens having a positive refractive power and having a convex surface facing the object side of the third lens group; the fourth lens group comprises one lens; each of the second lens group, the third lens group and the fourth lens group contains at least one aspherical surface; and the following relationships are satisfied: 1.8 less than L/CL1 less than 2.3, 0.2 less than f4/L less than 0.35, and 0.25 less than fw/f4 less than 0.4, where CL1 is an effective aperture of the lens positioned closest to the object in the first lens group, L is a distance from the lens positioned closest to the object in the first lens group to the image plane, f4 is a focal distance of the fourth lens group and fw is a focal distance at a wide-angle end.
According to such a zoom lens, the aberration can adequately be corrected by an interaction of the lenses. A compact and wide-angle lens having a high magnification can be obtained with a simple configuration. Furthermore, the sufficient back focus and a wide angle of view can be obtained, and the full length of the lens can be reduced.
It is further preferable in each of the above-mentioned zoom lenses that a zoom ratio is approximately 10 times. According to such a zoom lens, a compact and wide-angle zoom lens having a high magnification can be obtained.
Next, the video camera of the present invention is characterized by using any of the above-mentioned zoom lenses. According to the above-mentioned video camera, by using the zoom lens of the present invention, a small and lightweight video camera having high performance can be realized.
Next, the electronic still camera of the present invention is characterized by using any of the above-mentioned zoom lenses. According to the above-mentioned electronic still camera, since it uses the zoom lens of the present invention, a small and lightweight electric still camera having high performance can be realized.